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1
Two hours before mixing, spoon the fromage blanc into a cheesecloth-lined sieve set over a bowl and refrigerate. Discard the collected whey. The drained fromage blanc should be visibly firmer and hold its shape lightly on a spoon.
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2
Rub the finely grated lemon zest into the caster sugar with your fingertips until the sugar is pale yellow and fragrant — the oleo-saccharum releases the essential oils from the zest into the sugar crystals. Set aside. Line and blind-bake the tart shell in a 24 cm tin until the base is dry and just beginning to colour. The pastry should feel firm and dry to the touch at its centre.
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3
Whisk the egg yolks with the lemon sugar in a bowl until the mixture is thick, pale, and falls from the whisk in a slow, ribbon-like sheet — the ruban stage. This takes 3 to 5 minutes of sustained whisking. The ribbon should hold on the surface for two full seconds before dissolving.
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4
Fold the drained fromage blanc into the yolk mixture with a spatula, working until fully smooth. Add the crème fraîche and vanilla extract, and the cornflour if using, and fold again. The filling must be entirely free of lumps at this stage — any graininess from inadequately mixed fromage blanc will persist through baking and cannot be corrected. This is where the dish lives or dies.
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5
In a separate, scrupulously clean bowl, whisk the egg whites to soft peaks — the peaks should curl gently at their tips and the whites should not be dry or grainy. Fold one-third of the whites into the fromage blanc base to loosen it, then fold in the remainder in two additions, preserving as much air as possible. The finished filling should be pale, airy, and uniformly cloud-like.
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6
Pour the filling into the blind-baked tart shell, levelling it gently. Bake at 170°C for 35 to 40 minutes. The surface should develop golden freckles but must not colour uniformly. The tart is done when the outer rim is just set and the centre — approximately the inner 5 cm — yields a distinct, slow wobble when the tin is nudged. Remove from the oven at this point; residual heat will finish the set.
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7
Turn off the oven and leave the tart inside with the door ajar until the oven is cool. This gradual temperature reduction prevents the surface from cracking. Do not unmould before the tart is fully cooled. When ready to serve, dust with icing sugar and present at room temperature, where the contrast between the crisp pastry and the tangy, soufflé-like filling is at its most pronounced.